Foundations · Rafts & slabs on ground
Raft foundation calculator
Corner pressures with two-way eccentricity, kern check, elastic and consolidation settlement.
Standard: Rigid raft · Meyerhof effective area · Bowles settlement
Inputs in the worked example
The live calculator starts with these values; change any of them and every result, check and diagram updates instantly.
| Input | Value | Unit |
|---|---|---|
| Width B (x) | 12 | m |
| Length L (y) | 20 | m |
| Total service load | 24000 | kN |
| Eccentricity ex | 0.6 | m |
| Eccentricity ey | 0.8 | m |
| Allowable bearing | 150 | kPa |
| Soil modulus Es | 25 | MPa |
| Poisson's ratio | 0.3 | |
| Clay Cc (0 = skip) | 0.25 | |
| Clay thickness | 4 | m |
| Clay e0 | 0.9 | |
| σ'v0 at clay mid | 80 | kPa |
| Stress influence Δσ/q at clay mid | 0.6 |
Worked example — results
| Result | Value | Unit |
|---|---|---|
| Average q = P/A | 100 | kPa |
| q max | 154 | kPa |
| q min | 46.0 | kPa |
| Meyerhof B' × L' | 10.8 × 18.4 | m |
| q on effective area | 121 | kPa |
| Influence factor If (rigid) | 1.11 | |
| Immediate settlement | 48 | mm |
| Consolidation settlement | 128 | mm |
| Total settlement | 176 | mm |
- ✗ q max ≤ qa
- ✓ No uplift (resultant in kern)
- ✗ Total settlement ≤ 50 mm (typical raft limit)
Method, formulas and assumptions
- q = P/A (1 ± 6ex/B ± 6ey/L) — valid while the resultant is in the kern
- Meyerhof: B' = B − 2ex, L' = L − 2ey, q' = P/(B'L')
- Immediate: s = q B (1 − ν²) If / Es, If for a rigid rectangle (Bowles)
- Consolidation: s = Cc H/(1 + e0) log[(σ'0 + Δσ)/σ'0], Δσ = influence × q
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For preliminary design and education. Check every result against the current standard and your own engineering judgement before use.